2282307 attachment endpoint

Implementation of Attachment endpoint. 

What is not yet implemented/blockers: 
- For get request the value of "Content-Type" header is not correct in the response headers, it should be the mime type of the attachment.
- On put request the file should be attached to the entity as a new value for field_test_file.
Same here, we should use a Param Converter for the `$file_uuid` parameter, so that it automatically becomes `$file`.